It may not be the most obvious thing to do but if your getting design time errors in Blend – attach Visual Studio to it from your Silverlight project to get the exception. That was the good advice from Peter Blois when I asked for some help with the following error
As it turns out the error is a little strange as it was actually blowing up on a simple MessageBox.Show(“some string”)
There’s a good blog post on the Expression blog that details the debugging process and some additional tips when you get design errors. Read the post here.